Diffractive optical element and optical system having the same

ABSTRACT

In a diffractive optical element composed of three or more laminated layers and having a diffraction grating at each interface between adjacent layers, each even-number-th layer has a uniform thickness.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to diffractive optical elements,and more particularly to a diffractive optical element having such agrating structure that rays of a plurality of wavelengths or rays of aspecific wavelength band concentrate at a specific order (a designorder) of diffraction, and to an optical system having the diffractiveoptical element.

[0003] 2. Description of Related Art

[0004] Heretofore, as one of methods of correcting chromatic aberrationof an optical system, there is known a method of combining two glass(lens) materials which differ in dispersion.

[0005] In contrast to the method of reducing chromatic aberration bycombining glass materials, there is known another method, which isdisclosed in the optical literature, such as “International Lens DesignConference (1990)”, SPIE Vol. 1354, etc., and the specifications ofJapanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI6-324262 and U.S. Pat. No. 5,044,706. In the case of that method,chromatic aberration is corrected by means of a diffractive opticalelement which is provided with a diffraction grating for a diffractingaction and is disposed on a lens surface or a part of an optical system.That method is based on a physical phenomenon that the direction inwhich chromatic aberration arises for a ray of light of a referencewavelength becomes opposite between a refractive surface and adiffractive surface in an optical system.

[0006] Further, the diffractive optical element of such a type can bearranged to produce an advantageous correcting effect, like an asphericlens, on the aberration by varying the period of a periodic structure ofits diffraction grating.

[0007] Here, compared with a refracting action of rays of light, whileone ray of light remains one even after refraction at a lens surface,one ray of light is split into rays of a plurality of orders afterdiffraction at a diffractive surface.

[0008] Therefore, in using a diffractive optical element for a lenssystem, it is necessary to decide the grating structure in such a way asto cause a light flux of a useful wavelength region to concentrate at aspecific order (design order) of diffraction. With the light fluxconcentrating at the specific order, rays of diffraction light otherthan the light flux of the specific order have a low degree ofintensity. When the intensity becomes zero, the rays of diffractionlight would not exist.

[0009] In order to attain the above-stated feature, the diffractionefficiency of a ray of light of the design order must be sufficientlyhigh. Further, in a case where there are some rays of light havingdiffraction orders other than the design order, these rays are imaged ina place different from the imaging place of the ray of light of thedesign order, and thus appear as flare light.

[0010] For an optical system using a diffractive optical element,therefore, it is important to pay sufficient heed to the spectraldistribution of diffraction efficiency at the design order and thebehavior of rays of diffraction light of orders other than the designorder.

[0011]FIG. 11 shows a case where a diffractive optical element 1, whichhas a diffraction grating 3 and is composed of one layer on a base plate2, is formed on a surface of an optical system. In this case,diffraction efficiency for a specific order of diffraction is obtainedas shown in FIG. 12, which shows the characteristic of the diffractionefficiency. In FIG. 12, the abscissa axis of a graph indicateswavelength (nm) and the ordinate axis indicates the diffractionefficiency (%).

[0012] The diffractive optical element 1 is designed to have thediffraction efficiency become highest at the first order of diffraction(shown in a full line curve in FIG. 12) in the useful wavelength region.In other words, the design order of the diffractive optical element 1 isthe first order.

[0013] Further, FIG. 12 shows also the diffraction efficiency of adiffraction order near the design order, i.e., zero-order light andsecond-order light (1±1 order).

[0014] As shown in FIG. 12, at the design order, the diffractionefficiency becomes highest at a certain wavelength (540 nm) (hereinafterreferred to as a “design wavelength”) and gradually decreases at otherwavelengths. The lower portion of the diffraction efficiency obtained atthe design order becomes diffraction light of other orders and comes toappear as flare light. Further, in a case where the diffractive opticalelement is provided with a plurality of diffraction gratings, a drop indiffraction efficiency at wavelengths other than the design wavelengtheventually causes a decrease in transmission factor.

[0015] Diffractive optical elements having the structure capable oflessening the drop in diffraction efficiency are disclosed in JapaneseLaid-Open Patent Applications No. HEI 9-127321, No. HEI 9-127322, etc.According to the structural arrangement disclosed in Japanese Laid-OpenPatent Application No. HEI 9-127321, the diffractive optical element isformed by laminating two layers 4 and 17 as shown in FIG. 13 which is asectional view.

[0016] On the other hand, according to the structural arrangementdisclosed in Japanese Laid-Open Patent Application No. HEI 9-127322, thediffractive optical element has such a grating structure that threelayers 4, 17 and 6 are laminated as shown in FIG. 14. The thickness ofthe layer 17 which is interposed in between diffraction grating surfaces7 and 8 each of which is formed at a boundary face between two layers isnot uniform. Thus, the diffractive optical element has the diffractiongrating surfaces 7 and 8 which are formed at boundary faces betweendifferent materials. A high diffraction efficiency is attained byoptimizing a difference in refractive index between the materials oflayers disposed across each boundary and the depth of grating groovesformed in these layers.

[0017] In the above-stated diffractive optical element having a gratingstructure composed of a plurality of laminated layers, it is necessaryto make at a desired value a wavelength characteristic of a differencein refractive index between the layer materials disposed across (infront and in rear of) a diffraction grating surface. For example, in thearrangement disclosed in Japanese Laid-Open Patent Application No. HEI9-127321, one of the layers disposed across the diffraction gratingsurface must be made of a material which is of a high refractive indexand a low dispersion while the other layer must be made of a materialwhich is of a low refractive index and a high dispersion. Use ofmaterials for these layers is thus limited to this combination ofdifferent materials.

[0018] Further, in the case of the arrangement disclosed in JapaneseLaid-Open Patent Application No. HEI 9-127322, the kinds of layermaterials are increased to three kinds. The number of kinds ofselectable materials can be increased by varying the depth of gratinggrooves of the diffraction grating surfaces 7 and 8. The selectablelayer materials are, however, inevitably limited as long as thewavelength characteristic of a difference in refractive index betweenthese layer materials is used in arranging the diffractive opticalelement.

[0019] On the other hand, the materials usable across a boundary betweenlayers are limited, from a manufacturing viewpoint, to such materialsthat have good adherence to each other, nearly the same coefficients ofthermal expansion, and so on. It is also necessary that these materialsmust excel in workability for forming diffraction gratings.

[0020] The usable materials are thus limited not only in respect ofoptical characteristics but also from the manufacturing viewpoint.Therefore, it is not easy to find such optical materials that satisfyall of these conditions.

BRIEF SUMMARY OF THE INVENTION

[0021] It is an object of the invention to provide a diffractive opticalelement, or an optical system having the diffractive optical element,which is arranged to make selection of materials of layers relativelyeasy, to give a high diffraction efficiency and to effectively suppressflare light, etc.

[0022] To attain the above object, in accordance with an aspect of theinvention, there is provided a diffractive optical element composed ofthree or more laminated layers and having a diffraction grating at eachinterface between adjacent layers, in which each even-number-th layerhas a uniform thickness.

[0023] In accordance with another aspect of the invention, there isprovided a diffractive optical element composed of a plurality of layersmade of at least two kinds of materials of different dispersions (AbbeNumber νd) to enhance diffraction efficiency of a specific order over anentire useful wavelength region, in which, where the plurality of layersare counted in order as an i-th layer, a first diffraction gratingsurface is formed at a boundary between the first layer and the secondlayer, a second diffraction grating surface is formed at a boundarybetween the second layer and the third layer and an L-th diffractiongrating surface is formed at a boundary between the L-th layer and the(L+1)-th layer, and each even-number-th layer is a uniform-thicknesslayer having a uniform thickness over an entire area thereof.

[0024] In accordance with one mode of the diffractive optical elementaccording to the invention, the thickness of the uniform-thickness layeris greater than the depth of a grating groove of a diffraction gratingsurface formed at a boundary between the uniform-thickness layer and anadjacent layer.

[0025] In accordance with one mode of the diffractive optical elementaccording to the invention, the uniform-thickness layer has such athickness as to give a reflection preventing characteristic.

[0026] In accordance with one mode of the diffractive optical elementaccording to the invention, the uniform-thickness layer is made of aplastic optical material or an ultraviolet curable resin.

[0027] In accordance with one mode of the diffractive optical elementaccording to the invention, the useful wavelength region is a visiblespectrum.

[0028] In accordance with one mode of the diffractive optical elementaccording to the invention, the first layer is formed on a base plate,and the first layer and the base plate are made of the same material.

[0029] In accordance with a further aspect of the invention, there isprovided an optical system having the above diffractive optical element,which is, for example, an image forming optical system or an observationoptical system.

[0030] In accordance with a further aspect of the invention, there isprovided an optical apparatus or an electronic apparatus having theabove optical system.

[0031] These and other objects and features of the invention will becomeapparent from the following detailed description of preferredembodiments thereof taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0032]FIG. 1 is a front view showing essential parts of a diffractiveoptical element according to a first embodiment of the invention.

[0033]FIG. 2 is a sectional view showing essential parts of thediffractive optical element according to the first embodiment of theinvention.

[0034] FIGS. 3(A) to 3(D) are sectional views for explaining a method ofmanufacturing the diffractive optical element according to the firstembodiment of the invention.

[0035]FIG. 4 is a graph showing the diffraction efficiency of thediffractive optical element according to the first embodiment of theinvention.

[0036]FIG. 5 is a graph showing the diffraction efficiency of theconventional diffractive optical element.

[0037] FIGS. 6(A) to 6(E) are sectional views for explaining anothermethod of manufacturing the diffractive optical element according to thefirst embodiment of the invention.

[0038]FIG. 7 shows a modification in shape of the diffractive opticalelement according to the first embodiment of the invention.

[0039]FIG. 8 is a sectional view showing essential parts of adiffractive optical element according to a third embodiment of theinvention.

[0040]FIG. 9 schematically shows an optical system using a diffractiveoptical element according to a fourth embodiment of the invention.

[0041]FIG. 10 schematically shows an optical system using a diffractiveoptical element according to a fifth embodiment of the invention.

[0042]FIG. 11 is a sectional view showing essential parts of theconventional diffractive optical element.

[0043]FIG. 12 is a graph showing the diffraction efficiency of theconventional diffractive optical element.

[0044]FIG. 13 shows in a sectional view the grating shape of theconventional diffractive optical element.

[0045]FIG. 14 shows in a sectional view another grating shape of theconventional diffractive optical element.

[0046] FIGS. 15(A) and 15(B) are sectional views for explaining a methodof manufacturing the conventional diffractive optical element.

DETAILED DESCRIPTION OF THE INVENTION

[0047] Hereinafter, preferred embodiments of the invention will bedescribed in detail with reference to the drawings.

[0048]FIG. 1 is a front view showing a diffractive optical elementaccording to a first embodiment of the invention. Referring to FIG. 1,the diffractive optical element 1 is composed of a multi-layer part 3formed by laminating a plurality of layers on the surface of a baseplate 2. FIG. 2 is a sectional view of the diffractive optical elementtaken along a line A-A′ of FIG. 1. In FIG. 2, the diffractive opticalelement is illustrated in a shape exaggerated in the direction of thedepth of grating surfaces (diffraction grating surfaces) 7 and 8.

[0049] The sectional grating shape of the diffractive optical element 1in the first embodiment is composed of three layers, i.e., the firstlayer 4, the second layer 5 and the third layer 6. A first diffractiongrating surface 7 is formed at a boundary part between the first layer 4and the second layer 5. A second diffraction grating surface 8 is formedat a boundary part between the second layer 5 and the third layer 6. Thesecond layer 5 which is in contact with the two diffraction gratingsurfaces 7 and 8 is made to be a uniform-thickness layer having auniform thickness in the propagating direction of light over the entirearea of the diffractive optical element 1.

[0050] Further, since the first and second diffraction grating surfaces7 and 8 are formed through the uniform-thickness layer 5, the gratingshape of the first diffraction grating surface 7 is exactly the same asthat of the second diffraction grating surface 8. A feature of the firstembodiment lies in that the first and second diffraction gratingsurfaces 7 and 8 act as one diffractive optical element 1 throughout allthe layers thereof.

[0051] Here, a layer which has a diffraction grating surface on one sidethereof and a thickness of the material of which periodically varies, aseach of the layers 4 and 6, will be called a diffraction grating.

[0052] As described above, a diffractive optical element according tothe first embodiment of the invention has a grating structure formed bylaminating on a base plate a plurality of layers made of at least twokinds of materials of different dispersions. With a boundary between thefirst layer and the second layer, counting in sequence from the baseplate, assumed to be a first diffraction grating surface, a boundarybetween the second layer and the third layer assumed to be a seconddiffraction grating surface and a boundary between the L-th layer andthe (L+1)-th layer assumed to be an L-th diffraction grating surface,each of the even-number-th layers has a uniform thickness over the wholearea thereof, and the diffraction gratings are joined together througheach uniform-thickness layer.

[0053] Meanwhile, a structural arrangement similar to the shape of thediffractive optical element according to the invention was disclosed inJapanese Laid-Open Patent Application No. HEI 9-127321. However, thediffractive optical element disclosed in Japanese Laid-Open PatentApplication No. HEI 9-127321 uses a plurality of diffraction gratingsfor the purpose of allocating individual diffractive powers to them. Forthis purpose, each diffraction grating is arranged to independently haveits diffraction characteristic. In the case of the first embodiment ofthe invention, on the other hand, the two diffraction gratings 4 and 6are arranged to act integrally as one diffraction grating. Thediffraction grating according to the invention thus completely differsfrom the prior arrangement in size and material of the layer.

[0054] Further, the object of the prior arrangement cited above cannotbe attained by arranging the diffraction gratings in the same directionas the embodiment of the invention. In actuality, therefore, thediffraction gratings of the prior arrangement must be shaped to differin direction from the shape shown in FIG. 14. The diffractive opticalelement of the prior arrangement cited above thus completely differsfrom the diffractive optical element of the invention also in thispoint.

[0055] Next, the diffraction efficiency of the diffractive opticalelement according to the first embodiment of the invention is describedas follows.

[0056]FIG. 11 shows a transmission-type diffraction grating 3 of anordinary single-layer structure, which is to be used in air. In order toobtain a maximum diffraction efficiency for a design wavelength λ0 bythe diffraction grating of such a type, the optical path lengthdifference d0 between the crest and the trough of a diffraction gratingsurface 7 is required to be integer times as much as the designwavelength, as expressed below, for a light flux which isperpendicularly incident on the diffraction grating:

d0=(n0−1)d=mλ0  (1)

[0057] where n0 is a refractive index of the material of the diffractiongrating 3 at the wave length λ0, d is a grating thickness of thediffraction grating 3, and m is a design order of diffraction.

[0058] The basic concept of the diffractive optical element composed oftwo or more diffraction gratings, i.e., two or more layers, is the sameas that of the above-stated single-layer type diffraction grating. Inorder to have all the layers act as one diffraction grating, themulti-layer diffractive optical element is arranged as follows. Theoptical path length difference between the crest and the trough of thediffraction grating formed at each boundary between two layers isobtained, and the difference values thus obtained for all of the layersare added together to obtain a total sum. Then, the diffractive opticalelement is made to have the total sum become integer times as large asthe design wavelength.

[0059] Therefore, a conditional expression for the first embodimentshown in FIG. 2 becomes as follows:

(n01−n02)d−(n03−n02)d=mλ0  (2)

[0060] where n01 is a refractive index of the material of the firstlayer 4 at the wavelength λ0, n02 is a refractive index of the materialof the second layer 5 at the wavelength λ0, and n03 is a refractiveindex of the material of the third layer 6 at the wavelength λ0. In thecase of a diffractive optical element disclosed in Japanese Laid-OpenPatent Application No. HEI 9-127322, the diffraction gratings 4 and 6are arranged to differ in grating thickness from each other in theformula (2).

[0061] The first embodiment of the invention is characterized in thatthe two confronting gratings are arranged to be in shapes which areequal to each other. With the first embodiment arranged in this manner,the formula (2) becomes as follows:

(n01−n03)d=λ0  (3)

[0062] Then, with the diffraction efficiency assumed to be η, a phaseerror assumed to be φ0, being expressed as φ0=(n01−n03)d=mλ0), thediffraction efficiency ηcan be expressed as follows:

η=sin c ²[(n01−n03) d/mλ0−1]

=sin c²(φ0/mλ0)  (4)

[0063] where sinc(x) is a function which can be expressed assinc(x)=sin(πx)/πx.

[0064] Therefore, if the formula (3) is satisfied for the whole usefulwavelength region, the phase error φ0 becomes “0” in the formula (4), sothat the diffraction efficiency becomes maximum over the whole region ofuseful wavelengths, according to η=sinc² [0]=1.

[0065] Here, the refractive index of the second layer 5 is not includedin the formula (4). This means that the material of the second layer 5has no influence on the diffraction efficiency. Therefore, the materialof the second layer 5 can be selected from among materials having anyvalues of both the refractive index and dispersion.

[0066] According to the above feature of the invention, the materials ofthe first and third layers 4 and 6 are selected from among suchmaterials that have required optical characteristics, and the materialof the second layer 5 is selected from among such materials that arecomplementary to the shortcomings of the materials of the first andthird layers with respect to joining them directly with each other andcan be readily joined to both of the first and third layer materials.

[0067] The materials of these layers are selected as described below.

[0068] A case where glass is to be used for the first and third layersis first described as follows. In this case, a combination of glassmaterials that satisfies the condition for diffraction efficiency can beselected from among combinations equivalent to a combination of thematerials of a two-layer type diffractive optical element disclosed inJapanese Laid-Open Patent Application No. HEI 9-127321.

[0069] Meanwhile, in respect of manufacturing processes, a method forthe manufacture as shown in FIGS. 3(A) to 3(D) is conceivable. Thediffraction grating surface 7 is first formed by molding or the like onthe glass surface of the first layer 4 to obtain the diffraction grating4, as shown in FIG. 3(A). In this case, the base plate 2 and the firstlayer 4 are formed with the same material.

[0070] Then, the diffraction grating is formed with the third layer 6 inthe same manner as in the first layer 4, as shown in FIG. 3(B).

[0071] Next, the first and third layers 4 and 6 are joined togetherthrough the second layer 5, which is a uniform-thickness layer, as shownin FIG. 3(C). By the above process the final shape of the diffractiveoptical element is obtained as shown in FIG. 3(D). In the case ofjoining glass materials together, as shown in FIGS. 15(A) and 15(B), ithas been practiced to pour a second glass material 17 of the secondlayer into the first diffraction grating 4 of the first layer after thediffraction grating 4 is molded with a mold, as shown in FIG. 15(A), andthen to obtain a two-layer diffractive optical element by cooling thesecond layer, as shown in FIG. 15(B).

[0072] In the case of this conventional method, it has been a necessarycondition to use an optical glass material of a high fusing point forthe first layer 4 and a glass material of a low fusing point for thesecond layer 17. Therefore, even if there is such a combination ofoptical glass materials that gives a high diffraction efficiency inrespect of optical characteristic, the adoption of such advantageouscombination has been sometimes prevented by this manufacturingcondition.

[0073] To solve this problem, the first embodiment of the invention isarranged to permit use of either an ultraviolet curable resin or someother material that has a large difference in fusing point from glass,as the material of the uniform-thickness layer. The use of such amaterial permits two glass materials to be joined together. This methodwas disclosed also in Japanese Laid-Open Patent Application No. HEI9-127322.

[0074] The first embodiment is characterized by the provision of theuniform-thickness layer which permits a search for such opticalmaterials that give an adequate diffraction efficiency in respect to thefirst and third layers and a search in respect to the second layer for amaterial having such a property that is necessary for the manufacture.These searches for materials can be made independently of each other.The provision of the uniform-thickness layer facilitates the manufactureof a diffractive optical element with a combination of optical glass oftwo different kinds, which has been difficult because glass materialscannot be easily joined together. A diffractive optical element having ahigh degree of diffraction efficiency thus can be easily manufacturedaccording to the arrangement of the first embodiment.

[0075] Next, a case where glass is used for the first layer and aplastic material for the third layer is described as follows. In thiscase, a material which has an intermediate thermal expansioncharacteristic between glass and plastic materials is used for thesecond layer to join the first and third layers through the secondlayer. The use of such a material effectively solves a problem that,with glass and plastic materials directly joined together, theirboundary faces peel off from each other or fine cracks are produced dueto their expansion or contraction caused by heat.

[0076] Further, according to the above-stated manufacturing method, thefirst diffraction grating 4 and the second diffraction grating 6 are inprojected and recessed grating surface shapes which are perfectlyconversed. Therefore, the projected and recessed molds can be easilyreproduced by stamping with one mold used as an original. With theabove-stated manufacturing method employed, even in the event of somemanufacturing error, the grating surface shape of the first diffractiongrating 4 and that of the second diffraction grating 6 become identicalwith each other in inverted shapes in projection and recession.

[0077] This feature of the diffraction grating manufacturing method ishereinafter described in comparison with the arrangement of JapaneseLaid-Open Patent Application No. HEI 9-127322 in which the second layerhas a varying thickness.

[0078] Some deviation from a design value in grating thickness isconsidered to be inevitable. In the structural arrangement of theinvention, an optical glass material, BSM81 (nd=1.6400, νd=60.1), whichis a product of OHARA Co., is used for the first layer, an ultravioletcurable resin, C001 (nd=1.5250, νd=50.8), which is a product ofDAI-NIPPON Ink, Co., is used for the second layer, and a plasticmaterial PC (nd=1.5831, νd=30.2) is used for the third layer.

[0079] In this structural arrangement, the grating thickness of each ofthe diffraction gratings 4 and 6 formed at each boundary is 10.3 μm. Inthis instance, a manufacturing error of grating thickness is assumed tobe 3%. FIG. 4 shows in a graph the diffraction efficiency in each ofthese occurrences. In FIG. 4, a curve (i) indicates the diffractionefficiency obtained at the design value of grating thickness, and acurve (ii) indicates the diffraction efficiency obtained with thediffraction gratings manufactured to have a thickness thinner by 3% thanthe design thickness.

[0080] According to this manufacturing method, both the first and seconddiffraction gratings 4 and 6 are manufactured at a thinner thicknessvalue of 10 μm. The uniform thickness of the uniform-thickness layerwhich is a feature of the invention, therefore, can be maintained. Asapparent from the illustration, the diffraction efficiency changes onlyby 2% or thereabout even with the diffractive optical elementmanufactured with the error of 3% or thereabout by the above-statedmanufacturing method. Therefore, the diffractive optical element can beeasily manufactured by the above-stated method.

[0081] Next, as a case where different grating thicknesses are employed,the diffractive optical element is considered to have such aconstruction that the first layer is made of a plastic material PMMA(nd=1.4917, νd =57.4), the second layer is made of the plastic materialPC (nd=1.5831, νd=30.2), the third layer is air, the grating thicknessof the first diffraction grating 4 is 15.16 μm, and the gratingthickness of the second diffraction grating 6 is 3.34 μm. Themanufacturing error occurring in grating thickness is also assumed to be3%.

[0082]FIG. 5 is a graph showing the diffraction efficiency obtained inthis construction.

[0083] In FIG. 5, a curve (i) shows the diffraction efficiency obtainedwith the diffractive optical element manufactured at, the design valuesof grating thickness. A curve (ii) shows the diffraction efficiencyobtained with the first diffraction grating 4 manufactured to have itsgrating thickness thinner by 3% than the design value. A curve (iii)shows the diffraction efficiency obtained with the second diffractiongrating 6 manufactured to have its grating thickness thinner by 3% thanthe design value.

[0084] Both the curves (ii) and (iii) of FIG. 5 show that thediffraction efficiency drops by 10 to 15% or thereabout. In the case ofthe curve (iii), since the grating thickness is thin, the error inthickness is only 0.1 μm. Despite of such a slight change, thediffraction efficiency greatly varies. This clearly indicates that thestructural arrangement requires a considerably high degree of precisionfor the manufacture.

[0085] As apparent from the comparison made above, the arrangement ofthe first embodiment for forming the diffraction gratings at equalgrating thickness makes the manufacture of the diffractive opticalelement easier than in the case of the arrangement in which diffractiongratings are formed to have different thicknesses.

[0086] FIGS. 6(A) to 6(E) show another manufacturing method by which theabove-stated advantageous feature can be also attained. In the case ofthis method, a diffraction grating which is made of the glass materialof the first layer 4 is formed by molding with a mold, as shown in FIG.6(A). Then, by using the same mold 9 used for the first layer 4, asshown in FIG. 6(B), the second layer 5 is formed to have a uniformthickness as shown in FIG. 6(C).

[0087] Next, the third layer 6 is molded over the uniform-thicknesslayer 5, as shown in FIG. 6(D), to obtain a final shape as shown in FIG.6(E). This method is preferable, because it permits forming both thediffraction grating surfaces 7 and 8 with one and the same mold.

[0088] In the structural arrangement described above, the opticalcharacteristic of the second layer does not contribute to thediffraction efficiency. Therefore, no limitation is imposed on theoptical characteristic of the second layer. However, the diffractionefficiency can be enhanced further by using, for the second layer, amaterial having a high transmission factor within the useful wavelengthregion.

[0089] The thickness of the second, uniform-thickness layer can be setas desired as it does not contribute to the diffraction efficiency.However, an excessive thickness would make it hardly possible toconsider a plurality of laminated diffraction gratings as a singlediffractive optical element. Therefore, the thickness is preferably setto such a value that enables the diffractive optical element to have athin composite grating thickness composed of a plurality of layers.

[0090] The diffraction grating shape has been described by limiting itto a shape obtained within one period of diffraction grating. However,the diffraction efficiency is basically not affected by the pitch of thediffraction grating. In other words, the arrangement of the firstembodiment described above is applicable not only to the one-dimensionaldiffraction grating shown in FIG. 1 but also to all diffractive opticalelements having different grating pitches and/or grating shapes,including a diffractive lens shown in FIG. 7.

[0091] Further, in the case of the first embodiment, the invention isapplied to the diffractive optical element formed by arranging aplurality of diffraction gratings on the base plate 2. However, theadvantageous effects are also attainable by arranging diffractiongratings on a curved lens surface instead of a flat surface.

[0092] In the description of the first embodiment given above, thedesign order of diffraction is assumed to be the first order. However,the order of diffraction is not limited to the first order. The sameadvantageous effect of the invention can be attained for light of otherdiffraction orders, such as light of the second diffraction order, bydesigning a composite optical path length difference in such a way as toobtain a desired (design) wavelength at a desired diffraction order.

[0093] Next, a diffractive optical element according to a secondembodiment of the invention is described. In the first embodiment, thethickness of the second layer is not specified in particular. However,with no uniform-thickness layer used, a part of a light flux would cometo reflect if a difference in refractive index between layer materialsis large at a boundary between them. In such a case, the boundary facewould act like a reflection-type diffraction grating to generate lightof unnecessary diffraction orders, thereby causing flare light.

[0094] Therefore, in the second embodiment, the uniform-thickness layeris arranged to have such a thickness that effectively prevents lightfrom reflecting at the boundary face between different layer materials.The diffraction efficiency of the diffractive optical element thus canbe attained at the design order of diffraction with no light reflectedat unnecessary diffraction orders.

[0095] More specifically, with a refractive index of the material of thesecond layer at the wavelength λ0 assumed to be n02 and the thickness ofthe second layer be assumed to be d2, the second layer satisfies thefollowing relation:

n02×d2=m×(λ0/4),

[0096] where m is an integer.

[0097] In the second embodiment, two diffraction gratings are formedthrough a uniform-thickness layer. The number of diffraction gratingsis, however, not limited to two. Three or more diffraction grating partsmay be formed through uniform-thickness layers which are arrangedaccording to the invention as described above.

[0098]FIG. 8 shows in a sectional view a diffractive optical elementhaving three diffraction gratings, according to a third embodiment ofthe invention. Referring to FIG. 8, counting from the layer locatednearest to a base plate 2, these layers are laminated in the order offirst, second, third, fourth and fifth layers. First to fifthdiffraction grating surfaces are formed in the same order at respectiveboundaries between the layers. In this case, diffraction gratings areformed at the first, third and fifth layers. The second and fourthlayers are formed as uniform-thickness layers which represent thefeature of the invention. With these layers arranged in this manner, theshape of the diffraction grating surface formed between the first andsecond layers becomes the same as that of the diffraction gratingsurface formed between the second and third layers.

[0099] Generally, a diffractive optical element of a laminated-layerstructure composed of an L-number of layers, a 2h-th layer is arrangedto be a uniform-thickness layer to form a 2h-1)-th diffraction gratingsurface and a 2h-th diffraction grating surface in the same shape. Inthis case, there is obtained a relation of 2h≦L, where h represents apositive integer. Such arrangement gives the advantageous feature of theinvention even to a diffractive optical element of a multi-layerstructure.

[0100] A diffractive optical element according to a fourth embodiment ofthe invention is schematically shown in FIG. 9. FIG. 9 is a sectionalview of a photo-taking optical system of a camera or the like. Referringto FIG. 9, a photo-taking lens 10 includes therein a diaphragm 11 andthe diffractive optical element 1. An image forming plane 12 representseither a film or a CCD.

[0101] The use of the diffractive optical element which is composed oflaminated layers greatly improves the wavelength dependency ofdiffraction efficiency. The photo-taking lens, therefore, can bearranged to have little flare light and a high degree of resolution at alow frequency to ensure a high performance. Further, since thediffractive optical element according to the invention can be simplymanufactured, the photo-taking lens can be manufactured by massproduction at a low cost.

[0102] In the case of FIG. 9, the diffractive optical element 1 isprovided on the surface of a flat glass plate in the neighborhood of thediaphragm 11. However, the position of the diffractive optical element 1is not limited to this position. The diffractive optical element 1 maybe arranged on a curved lens surface. It is also possible to arrange aplurality of diffractive optical elements within the photo-taking lens.

[0103] The invention is applied to the photo-taking lens of a camera inthe case of the fourth embodiment. However, the same advantageous effectis attainable by using the diffractive optical element according to theinvention for a photo-taking lens of a video camera, an image scanner ofa business machine, a reading lens of a digital copying machine, etc.

[0104]FIG. 10 is a sectional view which schematically shows, as a fifthembodiment of the invention, an optical system using a diffractiveoptical element arranged according to the invention. The optical systemshown in FIG. 10 is an observation optical system of a binocular or thelike. The illustration of FIG. 10 includes an objective lens 13, thediffractive optical element 1, an image inverting prism 14, an eyepiecelens 15, and an evaluation surface (a pupil surface) 16.

[0105] The diffractive optical element 1 is provided for the purpose ofcorrecting chromatic aberration or the like taking place on an imageforming plane 12 of the objective lens 13.

[0106] The wavelength dependency of the diffraction efficiency of thefifth embodiment is greatly improved by the use of the diffractiveoptical element of the laminated structure. The objective lens whichuses the diffractive optical element according to the invention,therefore, has little flare light and a high resolving power at a lowfrequency to ensure a high performance. Besides, the diffractive opticalelement can be simply manufactured. Therefore, the observation opticalsystem according to the fifth embodiment of the invention facilitatesmass production to lower the manufacturing cost thereof.

[0107] In the fifth embodiment, the diffractive optical element 1 isdisposed in the neighborhood of the objective lens 13. The position ofthe diffractive optical element 1 is, however, not limited to this. Thesame advantageous effect is attainable, for example, by arranging thediffractive optical element 1 on the surface of the prism 14 or withinthe eyepiece lens 15. With the diffractive optical element 1 disposed onthe object side of the image forming plane 12, it serves to abatechromatic aberration caused solely by the objective lens 13. Therefore,in the case of an observation system for observation with the unaidedeye, it is preferable to have the diffractive optical element 1 set atleast between the objective lens 13 and the image forming plane 12.

[0108] In the case of the fifth embodiment, the invention is applied toa binocular. However, the same advantageous effect of the fifthembodiment is attainable by applying the invention to a terrestrial orastronomical telescope or to an optical viewfinder of a lens-shuttertype camera, a video camera, or the like.

[0109] According to the arrangement of each of the embodiments describedabove, the layers of the diffractive optical element formed bylaminating two or more layers on a base plate are appositely arranged toattain a high degree of diffraction efficiency. The arrangement makesselection of materials for these layers easier to facilitate themanufacture. Besides, the diffractive optical element according to theinvention is capable of retaining a high degree of diffractionefficiency and effectively suppressing the generation of flare light orthe like.

[0110] The advantages mentioned above are attained, because a pluralityof layers are laminated and joined together through a layer or layershaving a uniform thickness in accordance with the invention, in such amanner that the layers of uniform thickness are arranged to have suchoptical characteristics that contribute to enhancement of thediffraction efficiency while the uniform-thickness layer or layers havesuch a characteristic that contributes to facilitation of manufacturingprocesses.

[0111] The use of the diffractive optical element for a photo-takinglens according to the invention enables the photo-taking lens to bemanufactured at a low cost and to operate with a high degree ofaccuracy.

[0112] The use of the diffractive optical element for an observationoptical system according to the invention enables the observationoptical system to be manufactured at a low cost and to operate with ahigh degree of accuracy.

1. A diffractive optical element composed of three or more laminatedlayers and having a diffraction grating at each interface betweenadjacent layers, wherein each even-number-th layer has a uniformthickness.
 2. A diffractive optical element composed of a plurality oflayers made of at least two kinds of materials of different dispersionsto enhance diffraction efficiency of a specific order over an entireuseful wavelength region, wherein, where the plurality of layers arecounted in order as an i-th layer, a first diffraction grating surfaceis formed at a boundary between the first layer and the second layer, asecond diffraction grating surface is formed at a boundary between thesecond layer and the third layer and an L-th diffraction grating surfaceis formed at a boundary between the L-th layer and the (L+1)-th layer,and each even-number-th layer is a uniform-thickness layer having auniform thickness over an entire area thereof.
 3. A diffractive opticalelement according to claim 2 , wherein the thickness of theuniform-thickness layer is greater than the depth of a grating groove ofa diffraction grating surface formed at a boundary between theuniform-thickness layer and an adjacent layer.
 4. A diffractive opticalelement according to claim 2 , wherein the uniform-thickness layer hassuch a thickness as to give a reflection preventing characteristic.
 5. Adiffractive optical element according to claim 2 , wherein theuniform-thickness layer is made of a plastic optical material or anultraviolet curable resin.
 6. A diffractive optical element according toclaim 2 , wherein the useful wavelength region is a visible spectrum. 7.A diffractive optical element according to claim 2 , wherein the firstlayer is formed on a base plate, and the first layer and the base plateare made of the same material.
 8. An optical system using a diffractiveoptical element according to one of claims 1 to 7 .
 9. An optical systemaccording to claim 8 , wherein said optical system is an image formingoptical system.
 10. An optical system according to claim 8 , whereinsaid optical system is an observation optical system.
 11. An opticalapparatus or an electronic apparatus having an optical system accordingto claim 8 .